276 TEXT-BOOK OF ZOOLOGY depth of a few fathoms, semi-darkness prevails, consequently the eyes of fishes are generally remarkably large and their pupils very broad. (Compare with nocturnal animals, e.g., owls and cats.) In animals that live in the air the cornea is convex like a watch-glass, and, together with the aqueous humour between the cornea and the iris, has a strong refrac¬ tive action. In the eye of a fish, on the other hand, the refractive power of the cornea is about the same as that of the water, and the surface of the cornea is therefore flat. Eefraction in the eye of the fish is chiefly due to the lens, which is almost spherical. When a fish is boiled the lens becomes opaque, and then appears white. Eyelids which in other verte¬ brates protect the eye from external injury, especially from dust, are generally wanting in fishes. For this reason it is all the more important for them that the cornea does not project from the surface of the body. The ear of fishes is a very simple structure. Neither external ear, auditory passages, tympanum, or auditory ossicles are present, the sole functions of these organs being to catch the waves of sound and transmit them to the fiuids of the auditory labyrinth (see Part I., p. 13). The fish, however, can dispense with organs of this nature (compare with seal and whale), since the waves of sound, which are propagated in water with great ease, produce vibrations in the bones of the skull, which are trans¬ mitted to the fluid of the labyrinth. The nose does not, as in air-breathing animals, take any share in the function of respiration, hence it does not open into the interior of the oral cavity, but consists merely of two pits placed at the front end of the head, in which the water streams freely in and out. The whole of the integument functions as an organ of touch, the skin of the lips, however, being more specially sensitive to tactile impressions, and provided in many " bottom fishes" (sheat-fishes, tench) with fleshy tactile filaments, or " barbels." The sense of taste, which has its seat in the mucous membrane of the mouth, is but slightly developed. In many fishes (take a perch, for instance) a peculiar dark line may be observed running along the middle of each side of the body from head to tail. This is the so- called lateral line. Each of the scales in this line is per¬ forated by a tube opening on its surface and leading down to a longitudinal canal which runs underneath the lateral Scale of the ^^'^^' ^"^^ ^^ provided with peculiar bodies which have aU Lateral Line of the appearance of sense organs. Indeed, naturalists are wg°thb Canal. of opinion that they represent the organs of a sixth sense, unknown (and absent) in ourselves. Possibly this sense renders the fish cognizant of any excessive pressure of the water, since